This invention relates to strain sensing instruments for measuring weights and loads. More specifically, the invention relates to a bolt-on, temperature compensated strain sensor which can be used to provide load measurements and to a remote access system for conveying load data for containers.
Load measurement is required in industrial settings in order to determine whether a container is empty, partially full or full and therefore whether it needs to be serviced. The use of strain gauges in such applications is well known.
It is known to provide at least two strain gauges oriented at 90 degrees from one another and connected in a balanced bridge configuration in order to provide partial compensation for the mechanical deformation of the load structure induced by temperature changes rather than load. U.S. Pat. No. 3,116,469 to Wu teaches the placement of two strain gauges at 90 degrees to one another. The strain gauges are mounted on a shim, which is then secured to the structure to be measured by way of a single weld line. In such an arrangement, the principal strain axis does not pass directly through the strain sensitive gauge. In addition, welding the shim to the load structure when the gauges are already mounted on the shim risks inducing distortion and errors in the gauges.
It is also known in the art to supply a strain sensor comprising one or more strain gauges mounted on a plate and bolting the plate to the support structure of the container to be measured. U.S. Pat. No. 4,064,744 to Kistler teaches such a bolt-on strain sensor with increased sensitivity. Two strain gauges are mounted, one on each side of a slender metallic beam formed transversely within a mounting plate. The transverse beam is located along an axis defined by the two bolt attachment points of the strain sensor plate. The connections of the main body of the plate to the beam are such that compression or expansion of the plate will deflect the beam, which deflection is applied as a strain on the gauges mounted on the sides of the beam. While temperature compensation effects are discussed in the patent, Kistler acknowledges that the design may still be adversely affected by variations in temperature of the support structure.
One method adopted to overcome this deficiency is to install in a rosette pattern a pair of the bolt-on strain sensors taught in the ""744 patent. The pair of strain sensors are wired together in such a way that signals generated by temperature-induced strain substantially cancel each other out. This reduces errors caused by thermal effects. However, in order to ensure the accuracy of this system, the pair of separate sensors must be installed at precisely 90 degrees to one another, which can be difficult to accomplish in field conditions. Ensuring the requisite level of accuracy is also time consuming and costly.
The bolt-on strain sensor taught in U.S. Pat. No. 5,734,110 to Kosmal was designed to overcome this deficiency by forming the sensor in an L configuration, such that the strain gauges are already pre-set at 90 degrees to one another. Along each axis of the L are two mounting holes, with one mounting hole being common to each axis, for a total of three mounting holes. Strain gauges, each of the design taught in the ""744 patent, are placed along each axis.
Typically in the load measurement industry, the objective is to determine the weight or mass of material in a bin as precisely as possible. As a result, load measurement systems are made as sensitive as possible, including attempting to account for non-linearities, by means of additional sensors and circuit means. However, the present invention recognizes that for many applications, precise measurement of load status is not required. It is often sufficient for a party servicing a bin or a container, for example a dump truck driver, or a trucker attending from time to time at a sawmill to retrieve wood chip bins, to know only whether the bin is empty, full or partly full.
It is one object of the present invention to provide a load-sensitive, bolt-on load sensor that has effective partial temperature compensation.
It is another object of the invention to provide a load sensor that is easy to calibrate in the field.
It is a further object of the invention to provide a load sensor and data delivery system that enables remote operators to determine the load status of containers to facilitate the timing of servicing of the containers.
These and other objects of the invention will be appreciated by reference to the summary of the invention and the detailed description of the preferred embodiment that follows. It is noted that the foregoing objects are not necessarily met simultaneously in all aspects of the invention defined by each claim.
The load sensor according to the invention is designed to be mounted on a structure that is subjected to loading, for example a support beam for a load-carrying bin.
The sensor comprises a square plate having a mounting hole at each of its four corners. A first axis extends through diagonally opposed corners of the plate and a second axis extends through the other two diagonally opposed corners of the plate. A strain gauge is mounted along the first axis, preferably such that it also lies along the second axis.
In the preferred embodiment, the strain gauge has four strain sensing elements set in a square pattern. A first diagonally opposed pair of the elements is aligned such that their axis of sensitivity is parallel to the first axis. A second diagonally opposed pair of the elements is aligned such that their axis of sensitivity is parallel to the second axis.
The four elements of the strain gauge are connected in a standard temperature-compensating Wheatstone full bridge configuration.
The square plate is bolted onto the load structure such that the first axis corresponds to the principal strain axis along which strain is to be measured. For example, when measuring strain along a vertical support beam for a load-carrying structure, the plate is arranged on the beam in a vertical diamond configuration. Two diagonally opposed corners of the plate defining the first axis are positioned to lie along the vertical axis and the other two diagonally opposed corners defining the second axis lie along the horizontal axis. As a result, the two strain gauge elements whose axes of sensitivity are parallel to the horizontal axis act as temperature-compensating elements.
The assembly of the invention acts to effectively sense deflection of the load structure as a result of loading of the structure.
In an alternative embodiment, two dual-element strain gauges are used rather than a single multi-element gauge. A first strain gauge is mounted along a first axis extending through two diagonally opposed corners of the plate. A second strain gauge, used as a partial temperature-compensating element, is mounted along the axis extending through the other two diagonally opposed corners of the plate. The first and second strain gauges are again connected in a standard temperature-compensating Wheatstone full bridge configuration.
The inventors have found that the configuration of the invention significantly increases sensitivity of the sensor to strain along the principal strain axis while minimizing temperature or other stray effects, as compared to orienting the strain gauge and the plate in the same orientation.
A printed circuit board is mounted on four stand-offs so as to directly overlay the strain gauge elements and to sandwich them between the plate and the printed circuit board. The printed circuit board includes lead-throughs to extend leads from the strain gauge elements to traces on the distal surface of the board. A protective cap is secured with epoxy to the surface of the plate and encloses the strain gauge elements, leads and printed circuit board.
An on-site controller is installed in the general vicinity of the load structures being measured. The controller includes a micro-controller, a digital potentiometer, load status LEDs and calibration buttons. The digital potentiometer is connected to the bridge and controlled by the micro-controller to automatically balance the Wheatstone bridge circuit when a zero load calibration button is pressed. The potentiometer then balances the bridge and is held at the resulting potentiometer setting until calibration is once again initiated. The balancing of the bridge results in a substantially zero output voltage across the bridge. When a full load calibration button is pressed, the output voltage across the bridge is measured and is recorded in memory as the full load value. Subsequent output voltage readings are used to interpolate between the zero output voltage value and the full load value so as to deduce the load status of the container. Processing means are provided to manage load information and calibration for a plurality of sensors.
The assembly may be used in a system for allowing remote parties to determine the load status of bins for the purpose of determining whether to service the bins. In this application, the micro-controller averages the input from a plurality of sensors mounted on the several support beams of a bin. The results are compensated for wind, vibration and other effects to produce a final measured value of the bin load or of its fill status. The controller system further includes a modem to communicate bin load or fill status to a remote base station computer. The base station computer includes a web server having a user interface for making available over the Internet the bin load/fill status. Such information allows, for example, truckers attending at a sawmill to minimize wait times.
The ability to deliver to remote locations load status information will also be of value in applications involving load structures other than bins or containers, such as shelves, vehicles, etc.
In one aspect, the preferred embodiment of a load sensor assembly according to the invention comprises a substantially square plate with a hole in each of four corners of the plate for receiving a fastener. The plate has a first axis defined by a first pair of diagonally opposed holes and a second axis defined by a second pair of diagonally opposed holes. A strain a strain gauge secured to said plate.
In a further aspect, the strain gauge of the load sensor assembly has at least two uniaxial strain elements, where each of the uniaxial strain elements has an axis of sensitivity. The uniaxial strain elements are oriented within the strain gauge such that their respective axes of sensitivity are perpendicular to one another.
In a further aspect, the strain gauge is secured to the plate such that one of the axes of sensitivity is parallel to the first axis and the other of the axes of sensitivity is parallel to the second axis.
In yet another aspect, the strain gauge is mounted substantially centered on the plate.
In yet another aspect, the strain gauge has four uniaxial strain elements in a square pattern. A first pair of diagonally opposed elements have axes of sensitivity oriented parallel to the first axis, while a second pair of diagonally opposed elements have axes of sensitivity oriented parallel to the second axis.
In yet another aspect, the load sensor has a plurality of standoffs mounted on the plate such that the standoffs do not lie on either the first or second axes. A printed circuit board is mounted on the standoffs and overlays the strain gauge. The printed circuit board contains traces defining a Wheatstone full bridge configuration including the four elements of the strain gauge, and lead-throughs for establishing electrical connection between the traces and a plurality of leads emanating from the strain gauge. The leads extend from the strain gauge through the lead-throughs. The plate has substantially straight sides. A cover mounted on the plate to cover the standoffs, printed circuit board and strain gauge, the cover being oriented such that its corners lie along lines bisecting the straight sides of the plate.
In another aspect, the cover has a square cross section and is mounted to the plate to overlie the strain gauge such that each of its sides is perpendicular to either the first or second axis.
In another embodiment, the invention comprises a method of detecting strain in a structure comprising mounting a load sensor assembly on the structure by inserting fastening elements through the holes of the sensor plate such that the first axis lies along a principal strain axis along which strain is to be measured.
In another embodiment, a load sensor assembly according to the invention comprises a square plate having a hole in each of its four corners. Each hole is for receiving a fastener. A first uniaxial strain gauge is secured to the plate along a first axis defined between a first pair of diagonally opposing holes. A second uniaxial strain gauge is secured to the plate along a second axis defined by a second pair of diagonally opposed holes.
In another aspect, the load sensor assembly further comprises a digital potentiometer in parallel with the bridge configuration, at least one zero load calibration button and at least one full load calibration button, and control means responsive to the zero load calibration button to cause the potentiometer to adjust so as to balance the bridge.
In another embodiment, the invention comprises a load sensing system for providing information regarding the load status of a remote container. The system has a load sensor assembly comprising a square plate having a hole in each of four corners of the plate for receiving a fastener therethrough. The plate has a first axis defined by a first pair of diagonally opposed holes and a second axis defined by a second pair of diagonally opposed holes. A strain gauge is secured to the plate. Processing means are associated with the container for determining the load status based on an output of the strain gauge. There are also communication means for transmitting load status information to a remote server, database means associated with the remote server for correlating the load status to one of a plurality of remote individual containers, and a web server for displaying the load status information.
In another aspect, the load assembly of the load sensing system further comprises a plurality of standoffs mounted on the plate, so as not to lie on either the first or second axes. A printed circuit board is mounted on the standoffs and directly overlays the strain gauge. The printed circuit board contains traces defining a Wheatstone full bridge configuration including the strain gauge, and lead-throughs for establishing electrical connection between the traces and a plurality of leads emanating from the strain gauge. The leads extend from the strain gauge through the lead-throughs. A cover is mounted on the plate to cover the standoffs, printed circuit board and strain gauge, and is oriented such that its corners lie along lines bisecting the straight sides of the plate.
In another embodiment, a load sensing apparatus comprises at least two strain sensing elements and a Wheatstone bridge circuit comprising the at least two strain sensing elements. A digital potentiometer is connected to the bridge circuit. There are also processing means for causing the digital potentiometer to adjust so as to balance the Wheatstone bridge.
In another aspect, the Wheatstone bridge circuit of the load sensing apparatus comprises at least four strain sensing elements rigidly associated with a mounting plate, wherein a first pair of the elements are oriented on the mounting plate along a principal strain axis thereof, and a second pair of the elements are oriented on the mounting plate perpendicular to the principal strain axis.
In another aspect, the plate of the load sensing apparatus is square and the principal strain axis is defined as an axis extending between two opposed corners of the plate.
These and other aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiment and to the claims, which are incorporated in this summary in their entirety by reference.